A conventional butterfly valve is a type of flow control device used to manage a flow of fluid through a section of pipe. The typical butterfly valve includes a hollow cylindrical housing, a flat, circular plate, and a rotatable shaft. The plate is disposed within the housing at a point intermediate to the length of the cylindrical housing and secured to a lower portion of the rotatable shaft. An upper portion of the rotatable shaft is coupled to an actuator. During operation, movement of the actuator is translated to the plate. As the actuator moves, the plate is rotatably positioned within the housing anywhere from perpendicular to parallel to the direction of the flow of fluid through the valve. When the plate is perpendicular to the fluid flow direction, the valve is closed and the fluid is restricted from flowing through the valve. In contrast, when the plate is parallel to the fluid flow direction, the valve is fully open and the fluid flow through the valve is at its maximum. By moving the plate between perpendicular and parallel positions, the valve can be partially opened to provide metered fluid flows.
Conventional butterfly valves either have a clearance fit between the plate and the flow path to avoid wear, which results in high leakage rates, or have contact between the plate and the flow path. Since the radial stiffness of both the plate and the housing are high, a butterfly valve with too low a clearance fit may require high actuation forces and risks jamming and/or high wear rates at any contacting points. The high wear rates of the contacting surfaces result in undesirable leakage of the valve in the closed position. To replace a worn plate and prevent further leakage, the valve shaft is removed to provide access to the plate. Removing the valve shaft is often an arduous and time consuming task and, therefore, changing a worn plate can be a difficult and lengthy process.
Most butterfly valves, especially butterfly valves rated for high operating pressure, are also known to have somewhat lower flow rates (i.e., Cv rates) compared to, for example, ball valves since the flow path is somewhat obstructed by the plate and the shaft. A low flow rate often requires a larger valve for a given application, which may not be practical in terms of cost, available space, and available actuation torque.
Moreover, standard butterfly valves sometimes require high actuation torque to move the plate, especially from a closed position to an open position. Such high actuation torque is due to, for example, high valve closed seating forces, metal galling between the plate and the housing, a high spring preload needed to bias the valve to the closed position upon a loss of actuator power, high flow induced torque, and the like. As a result of requiring high actuation torque, larger and more costly actuators must be employed to open and close the valve.
A standard butterfly valve has the flow passage extending beyond the plate in both directions. Hence, the geometry modifications to the plate and shaft assembly in order to reduce flow induced torque are limited to locations inside the flow passage diameter when the valve is at any position. These modifications provide only minimal reduction of flow induced torque and may reduce the flow area of the fully open valve.